The update is so small that the changelogs for both the Pixel and Pixel XL only have five words: "This update improves Wi-Fi connectivity." It's almost definitely just a fix for a small bug that Google caught before the public release of the phones. It's likely that phones from the Google Store will also receive this update, if they don't ship with it already.

Currently, review units from the Google Store are on build NAE63P, and the firmware that was leaked a few weeks ago was build NDE63H. Verizon has already started shipping their (non-bootloader-unlockable) phones. For those of you who pre-ordered, perhaps David's review will keep you at ease until your very own Pixel arrives on your doorstep.